Microencapsulation is a process by which small, tiny solid particles, liquid droplets, or gas bubbles are surrounded or coated with a continuous film of polymeric material. The core of the microcapsule can also be referred as active agent, active core, core material, fill, internal phase, nucleus, and payload and the coating material used to coat the outer surface of the microcapsule is also called as coating, membrane, shell or wall. The coating material may be selected from the group consisting of organic polymer, hydrocolloid, sugar, wax, metal, or inorganic oxide. Microencapsulated substances may resolve many problems in various industries which are often having unnoticeable limitations like high reactivity, high volatility, pressure sensitivity, fast releasing, environmental and human toxicity, solubility and oxygen sensitivity which can be easily eliminated by incorporating microencapsulated additives.
Microencapsulation of amine can solve many problems faced in above applications. From the point of formulation and use, coating material can be hot melt, solvent based, emulsion based (usually aqueous), or pressure sensitive. However some high performance coatings such as epoxy resin consists of two chemical species which react upon mixing. As a result, they need to be packaged in two separate containers.
Having such coating formulation in two separate containers and then mixing them in a proper way and material ratio, is a problem for paint companies and also for the applicator which can be overcome by microencapsulating one of the reacting components and mixing these microcapsules in desired composition with the other reacting component formulation. One of such component is polymeric amine or termed as polyamine which has at least two amine functionality. Polyfunctional amines form an important class of epoxy hardeners.
U.S. Pat. No. 6,489,405 describe such epoxy composition wherein epoxy hardener (polyoxypropylenediamine:diethylenetriamine 95:5) 60 parts is mixed with 100 parts by weight of epoxy resin and cured for 3 hours at 80° C. Microcapsules containing reactive amine are found to be effective in self-healing epoxy composites ((Polymer Preprints, 2008, 49(1), p 963, Macromolecules, 2010, 43, p 1855-1859).
Microcapsules containing trioctylamine are useful in removing Zn(II) and Cu(II) from chloride aqueous solutions (Fonseca C. et al; J. Chilean Chem. Soc. 2010, 55, p 408).
The use of microcapsules in coating materials such as paint is well known in the literature. (Joaquln et al, US 2008/0039551; Jadhav et al, Int. J. Indust. Chem., 2013, 4, p 31; Shukla et al, US 2007/0053950; Pigment & Resin Technology, 1998, 17(4)).
Pressure sensitive microcapsules containing curing agent find applications to produce one pack system. (Wang et al, High Performance Polymers, 2012, 24(8), p 730; Liao et al, Chinese Sci. Bull., 2011, 56, p 439; Ryuji et al, U.S. Pat. No. 5,726,222). Though literature describing use of microcapsules containing amine is available, it is restricted to water-insoluble amine and/or amine-adduct.
Water soluble amines find applications in various fields as listed at http://www.dow.com/amines/apps/ viz. adhesives, agriculture, asphalt additives, bleach activator, cement, chelating agents, coatings, surfactants, textile/textile additives, urethane chemicals/foams and wood treating detergents/cleaner/fabric, drainage aids, epoxy curing agents, fungicides, gas treating, hydrocarbon purification, ion exchange resins, papers, paper wet strength resins, petroleum chemicals, pharmaceuticals, personal care, photographic chemicals, printing inks (U.S. Pat. No. 4,729,792), rubber/rubber processing aids (US20030165682), corrosion inhibitors (U.S. Pat. No. 5,922,652) diagnostics (U.S. Pat. No. 4,940,645), softeners, (US 20140017307), Fuel/lube oil additives (US 2005/0160662) etc.
One of the water soluble amine is polyaziridine having carboxylic acid functionality. It has applications in coatings, ink and adhesives to promote physical and chemical properties. However, polyaziridines have drawback of limited pot life in water due to reactive aziridine rings.
There is need to have pressure sensitive microcapsules containing water-soluble amine which are useful in applications such as laundry, agriculture, pharmaceuticals, cosmetics, construction materials, food, coating materials and other uses as listed above. In the past, there have been attempts to prepare microcapsules containing water soluble substances and amines. However these water soluble substances are not reactive compounds such as amines. There are reports in the literature which describe only microencapsulation of water-insoluble amines. Some reports mention the need of converting water-soluble amine to its derivative such as salt or adduct which becomes water-insoluble and then follow the encapsulation. When such amines (in salt or adduct form) are released, will need some external trigger to convert these amine derivatives to neat amine. Interfacial or dispersion polymerization is considered as industrially feasible microencapsulation process.
Other microencapsulation processes involve spray drying, solvent evaporation etc. However such processes involve use of large amount of solvents and/or multiple emulsions and thus are not suitable for making microcapsules which are intended to be used in various applications like adhesives, self-healing composites, agriculture, pharmaceuticals etc.
U.S. Pat. No. 5,401,505 (Bradley et al) describes preparation of microcapsules by interfacial polymerization, wherein aziridine is used as wall forming monomer and not as a core material (liquid fill). Bradley et al mentions that the fill (core material) within microcapsules will be a hydrophobic or hydrophilic liquid or solid wherein the invention has drawbacks such as use of sulfuric acid to protonate aziridine, large particle size 500 to 2300 microns and release of water along with hydrophilic core material.
U.S. Pat. No. 8,318,060 (Donal et al.) teaches conversion of amine to amine adduct which is water in-soluble then microencapsulated by free radical polymerization in aqueous medium. However the process does not describe microcapsule containing neat water soluble amine.
David et al (Polymer Preprints, 2008, 49(1), p 963, Macromolecules, 2010, 43, p 1855-1859) describe microencapsulation of polyfunctional amine. i.e. DEH-52 which is a diethylenetriamine (DETA) adduct of DGEBA (diglycidyl ether of bisphenol A) and excess DETA. Jun et al (ACS Macro Lett. 2014, 3, 976-980) reported encapsulation of tetraethylenepentamine by an emulsion-templated in situ polymerization using inverse Pickering emulsion. However, the resultant microcapsules encapsulate tetraethylenepentamine along with water. Thus these articles do not teach encapsulation of neat amine. Henghua et al. (Polymer, 2012, 53(2) p 581-587) have reported microcapsules containing aliphatic polyamine (EPIKURE 3274) which are prepared, first by preparing hollow urea-formaldehyde microcapsules (with 9-18% yield) which are then loaded with amine by vacuum infiltration for several hours. Thus the process involves two steps and produce microcapsules having an average diameter of 117±32 microns.
Hu, Jian-qing et al (Journal of Central South University of Technology, 18, (2) p-337-342) and JP 2004008015 described manufacture of microcapsules containing water soluble substances such as polyfunctional aziridine. However the drawback of this method is that microcapsules are prepared by multiple emulsion-solvent evaporation method and thus very less core content of ˜22% is achieved and that any traces of solvent in microcapsules are not acceptable when microcapsules are intended to be used in coating application. Further Japanese patent JP 03182520A teaches preparation of microcapsules containing amine curing agents having water solubility less than 10%. Thus amines which are freely soluble in water cannot be encapsulated by the method described.
U.S. Pat. No. 5,962,003 (Shukla et al.) have reported preparation of microcapsules containing water-soluble pesticide using polyurethane as wall material. The process involves interfacial reaction between diol and diisocyanate. When amine is to be used as core material the process would involve first emulsion of mixture of amine and diol in aliphatic hydrocarbon followed by addition of diisocyanate. In this case as reactivity of isocyanate (NCO) with amine is more than that with OH group of diol and the reaction between amine and NCO would also take place and microcapsules would contain mixture of diol and amine. When this process was followed to microencapsulate polyaziridine it resulted in irregular particles in the range of 20 to 300 microns.
Nishijima et al. (JP 48076935) discloses process for the preparation of microcapsules containing amine curing agent involving water as continuous phase. The process involves emulsifying amine and organic solvent solution in water followed by addition of water-soluble epoxy compound, the said process is carried out in aqueous phase and there are great chances of organic solvent being encapsulated in microcapsules along with amine.
U.S. Pat. Nos. 4,592,957 and 4,517,141 (Dahm et al) relates to microcapsules having a hydrophobic core and capsule walls formed by the reaction of water-soluble polyisocyanate bisulphite adduct with water-insoluble polyamines. Diethyl tolylene diamine (dye-precursor), which is water-insoluble amine, is used as core (liquid fill) and also wall forming monomer.
Further, WO 98/57737 teaches a method for microencapsulating water-soluble or water-dispersible or water-sensitive material, which is particularly useful for encapsulating breaker materials which are employed to break fracturing fluids. The method involves use of toluene in continuous phase. As amines such as polyaziridine, DETA and TETA are miscible in toluene, water-soluble amines cannot be encapsulated by WO'737 method.
U.S. Pat. No. 6,489,405 describe epoxy composition, wherein epoxy hardener (polyoxypropylenediamine:diethylenetriamine 95:5) 60 parts is mixed with 100 parts by weight of epoxy resin and cured for 3 hours at 80° C. EP 0590975 A1 describes one component type epoxy resin which is bisphenol A diglycidyl ether or bisphenol F diglycidyl ether, which comprises a) 100 parts by weight epoxy resin b) 5-100 parts by weight of a reactive diluent and c) 1-100 parts by weight of a latent curing agent, which is a microencapsulated amine.
Wang Fang et al describes preparation of microcapsules using polyetherimide as wall material by solvent evaporation technique. The microencapsulation process involves water as continuous phase and thus the process is not useful for preparation of microcapsules containing water soluble amine. (High Performance Polymers, 2012, 24(8), p 730).
Microcapsules containing reactive amine are found to be effective in self-healing epoxy composites ((Polymer Preprints, 2008, 49(1), p 963; Macromolecules, 2010, 43, p 1855-1859).
U.S. Pat. No. 5,849,412 describes encapsulated gel network comprises non-aqueous reactive material selected from the group consisting of a polyol, an isocyanate, a multifunctional amine, an organometallic, an acrylate, an acid, an acid anhydride and an acyl halide and diisocyanates or polyisocyanates or mixtures thereof.
WO2007081350 describes an encapsulated colorant compositions having a high encapsulation efficiency, wherein capsule core comprising at least one colorant, and a capsule shell, comprises reaction products of: (a) at least one di-, oligo- and/or polyisocyanate, and (b) at least one polyfunctional amine selected from the group consisting of polyvinylamines, polyethylenimines and polyoxyalkylenamines; (c) optionally, one or more alkyldiamines having 2 to 10 carbon atoms. The average particle size of said encapsulate less than about 5 microns.
WO2013101889 discloses a composition with increased release comprising; an encapsulated active and one or more metal additive, wherein encapsulated active is an amine resin. Further the aqueous dispersion of the encapsulated active is between 0.1 micron and 80 micron.
U.S. Pat. No. 5,191,012 describes aqueous dispersion of an encapsulated polyisocyanate which contains unreacted isocyanate groups which is prepared by dispersing in water a polyisocyanate and reacting the dispersed polyisocyanate with a polyamine having primary and/or secondary amino groups and a molecular weight of 400 or less in an amount sufficient to provide an equivalent ratio of amino groups to isocyanate groups of at least 0.05:1.
It may also be noted that, Sam Popwell et al. in Polymer Preprints 2005, 46(1), 735 describes preparation and analysis of initiator-core polyurea microcapsules using pentamines and diamines as shell components, wherein the isocyanate to primary amine ratio affects the stability of polyurea shell microcapsules. Further the core solution comprises cumene hydroperoxide, polyfunctional isocyanates, polyvinyl alcohol solution, aqueous polyamine. The microcapsule formed is separated using fumed silica. The particle size of core is ranging from 150-300 micron in diameter.
Recently microcapsules containing chemical reagent have shown to be useful to perform chemical synthesis as reported in Accounts in Chemical Research, 46(2), 2013, p 327-338 by Ashley R. et al. In many chemical reactions reagent compatibility plays critical role when oxidants/reductants or nucleophile/electrophile steps are coupled together for example, amine catalyst and nickel complexes are known to be incompatible because of their tendency to chelate and deactivate each other. Because of the fact that most successful catalyst for the Henry elimination are amine and especially polyamine, Tyler et al. have studied microcapsules containing polyethyleneimine (PEI). They have demonstrated that two step reaction capable of converting both aromatic and aliphatic aldehydes to their corresponding Michael addition adduct in a single vessel is possible by use of microcapsules containing PEI (Sarah L. Poe, Murris Kobaslija and D. D. Tyler Mcquade, J Am Chem Soc., 129, 2007, p 9216-9221). Tyler et al have prepared these PEI microcapsules, wherein PEI is encapsulated as solution in methanol, dimethyl formamide (DMF) and formamide. The same authors have reported microcapsules containing coumarin-1 wherein PEI is used as wall forming monomer (Murris Kobaslija and D. D. Tyler Mcquade, Macromolecules, 39, 2006, p 6371-6375). In this report too when microcapsules are prepared by oil in oil emulsion, disperse phase contains solvents listed above (Methanol, DMF and Formamide). Also Coumarin-1 is sparingly soluble in water (WO 2008127423 A2). Thus they do not teach how to encapsulate water-soluble neat amine.
The prior art reveals that the preparation of amine microcapsules is accompanied by cumbersome process steps such as converting into an insoluble salt or adduct and then encapsulated, large particle size of microcapsule, less core content, release of water along with core material and instability etc. Further coating system employed in the art is usually a two pack system of epoxy and hardener, wherein encapsulation of water soluble amine is not addressed.
One of the water soluble amine is polyaziridine having carboxylic acid functionality. It has applications in ink and adhesives to promote physical and chemical properties. However, polyaziridines have drawback of limited pot life in water due to reactive aziridine rings.
Qi Li et al have described dual component microcapsules of epoxy resin and curing agent for self healing epoxy composite (Composites Part B, Vol 55, 2013, pp 79-85). They have prepared microcapsules containing DGEBA (diglycidyl ether of bisphenol A) epoxy resin and microcapsules containing water soluble amine namely polyetheramine. Microcapsules containing this amine are prepared by solvent evaporation method using poly(methylmethacrylate) as encapsulating polymer and using water as continuous phase. It has been reported and known that the major problem with this solvent evaporation technique is a poor encapsulation efficiency of moderately water soluble and water soluble substances which partitioned out from the organic disperse phase in to the aqueous continuous phase (Shashank Tiwari and Prerana Verma, Int. J. Pharm and Life Sci. Vol. 2(8), 2011, pp 998-1005). In the above said reference of Qi Li, though they planned 80% loading of polyether amine in the microcapsules, they could get microcapsules containing only 20% amine.
Thus the prior art fails to provide polymer microcapsules containing neat water-soluble polyamine with core loading more than 28% having particle size less than 100 microns, for use in coating application-like paints and a process, where these microcapsules are produced by in-situ polymerization method carried out in non-aqueous medium.
Thus the preparation of microcapsules containing water soluble substance by solvent evaporation method using water as aqueous phase is not a feasible process to get microcapsules with higher active loading (up to 80%) and with high yield. The inventors have explored this solvent evaporation method using w/o/w emulsion using water as continuous phase and also by o/o emulsion using non-aqueous (paraffin oil) continuous phase to achieve higher loading of amine and yield but the experiments resulted in very poor yield (20-50%), very low amine content (less than 1%) and/or big polymer lump.
Therefore, the present inventors have developed the one pack system of microcapsule composition containing water soluble neat amine with dual characteristics of active ingredient and microcapsule wall forming agent, along with polymeric surfactant/stabilizer; polyisocynate; fumed silica. Further the neat amine gets released in substantial quantity from the microcapsules without any formation of derivatives, salts and adduct thereof. Microcapsules containing water soluble amine of the present invention has potential application in self-healing composites, pesticides, asphalt additives, chelating agents, corrosion inhibitors, detergents, epoxy curing agents, fungicides, lube oil additives, papers, pharmaceuticals, personal care, photographic chemicals, printing inks, textile and as reagent in various chemical reactions etc. The microcapsule of the instant invention is a cost-effective process for preparation and finds industrial applicability.